Method of manufacturing solid-state imaging device

ABSTRACT

A solid-state imaging device with an improved heat release-ability for releasing a heat generated in the amplifier unit of the solid-state image sensing element. The solid-state imaging device  10  of the present invention includes an elongated substrate (molded case  18 ), a metallic layer  16  exposed in a surface of the molded case  18  and extending along an elongating direction of the molded case  18 , and an elongated solid-state image sensing element  20  mounted on the metallic layer  16 , in which a thickness in a region of a metallic layer  16  right under an amplifier unit of the solid-state image sensing element  20  is larger than thicknesses in other regions of the metallic layer  16.

This application is a continuation of co-pending application Ser. No.11/970,546 filed on Jan. 8, 2008, which claims foreign priority toJapanese patent application No. 2007-012638. The content of which isincorporated hereinto by reference.

BACKGROUND

1. Technical Field

The present invention relates to a solid-state imaging device includinga solid-state image sensing element having a plurality ofphoto-transistors that are formed to be linearly arranged.

2. Related Art

A typical example of a conventional solid-state imaging device isdescribed in Japanese Patent Laid-Open No. H6-163,950 (1994). FIG. 7shows a cross-sectional view along a width direction of a solid-stateimaging device described in Japanese Patent Laid-Open No. H6-163,950.

A solid-state imaging device 110 includes an elongated solid-state imagesensing element 120 in substantially central portion in a molded case118, which includes a metallic layer 112 called an “island”. Thesolid-state image sensing element 120 is mounted on the metallic layer112. The surface of the solid-state image sensing element 120 isprovided with a bonding pad, which is not shown here. A surface of aninner lead 134 is exposed within the molded case 118. The bonding pad iselectrically coupled to inner lead 134 through bonding wires 140.Further, the inner lead 134 extends through and beyond the molded case118 to be coupled to an outer lead 130 outside of the molded case 118.An upper opening of the molded case 118 is encapsulated by a transparentplate 122.

Such type of structure is also described in Japanese Patent Laid-OpenNo. H7-86,542 (1995), Japanese Patent Laid-Open No. H7-161,954 (1995),Japanese Patent Laid-Open No. H11-330,285 (1997) and Japanese PatentLaid-Open No. 2001-68,578.

Further, Japanese Patent Laid-Open No. 2000-228,475 describes, as shownin FIG. 8, a solid-state imaging device employing a lead frame having ametallic layer 112 of larger dimension in a region right under anamplifier unit of a solid-state image sensing element. It is describedthat a heat from the amplifier unit of the solid-state image sensingelement can be quickly released according to such configuration.

However, the conventional technologies as described above face problemsto be solved in the process of releasing heat that has been generated inthe amplifier unit of the solid-state image sensing element.

In recent years, a downsizing of a solid-state imaging device isrequired, and developments for achieving more dense arrangement ofpixels and a miniaturization of solid-state image sensing elements areproceeded. Such developments causes an increased amount of heatgenerated in the amplifier unit of the solid-state image sensingelement, leading to an elevated temperature of the solid-state imagesensing element and uneven temperature distribution of the pixels. Thisadversely affects a quality of an image read by the solid-state imagingdevice, causing a considerable deterioration in the reliability of thesolid-state imaging device. Therefore, there is a need in the industryfor reducing the temperature of the solid-state image sensing elementduring the operation by releasing heat from the amplifier unit of thesolid-state image sensing element with an improved efficiency to providean uniform temperature distribution of pixel, thereby achieving astabilized imaging function of the solid-state image sensing element.

More specifically, the present invention is directed to providing asolution to new and novel issue that is arisen due to a downsizing ofsolid-state imaging devices, and the issue is solved by achieving animproved efficiency in releasing a heat generated in the amplifier unitof the solid-state image sensing element.

SUMMARY

In one embodiment, there is provided a solid-state imaging device,comprising: an elongated substrate; a metallic layer exposed to asurface of the substrate and extending in an elongating direction of thesubstrate; and an elongated solid-state image sensing element mountedover the metallic layer, wherein a thickness in a region of the metalliclayer right under an amplifier unit of the solid-state image sensingelement is larger than thicknesses in other regions of the metalliclayer.

Since the metallic layer is formed to be thicker in the region of thesolid-state image sensing element right under the amplifier unitaccording to the present invention, a heat generated in the amplifierunit can be released with an improved efficiency. This allows providinguniform temperature distribution of pixels, thereby achieving astabilized imaging function of the solid-state imaging device.

According to the present invention, an improved heat release-ability forreleasing a heat generated in the amplifier unit of the solid-stateimage sensing element is provided, so that the solid-state imagingdevice, which is capable of providing an improved imaging function, canbe provided.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, advantages and features of the presentinvention will be more apparent from the following description ofcertain preferred embodiments taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a schematic plan view of a solid-state imaging device of anembodiment according to the present invention;

FIG. 2 is a cross-sectional view of the solid-state imaging device shownwith FIG. 1 along line a-a;

FIG. 3 is an enlarged cross-sectional view of the solid-state imagingdevice shown in FIG. 2 in first embodiment;

FIG. 4 is a schematic plan view of a lead frame employed formanufacturing a solid-state imaging device according to the presentembodiment;

FIG. 5 is a table including experimental results that shows arelationship between a thermal resistance of the solid-state imagingdevice and a thickness of the second metallic layer;

FIG. 6 is an enlarged cross-sectional view of the solid-state imagingdevice 10 shown in FIG. 2 in second embodiment;

FIG. 7 is a cross-sectional view of a conventional solid-state imagingdevice along a width direction; and

FIG. 8 is a schematic plan view of a lead frame employed formanufacturing a solid-state imaging device disclosed in a related priordocument.

DETAILED DESCRIPTION

The invention will be now described herein with reference toillustrative embodiments. Those skilled in the art will recognize thatmany alternative embodiments can be accomplished using the teachings ofthe present invention and that the invention is not limited to theembodiments illustrated for explanatory purposed.

Exemplary implementations according to the present invention will bedescribed in reference to the annexed figures. In all figures, anidentical numeral is assigned to an element commonly appeared in thefigures, and the detailed description thereof will not be repeated.

First Embodiment

FIG. 1 shows a schematic plan view of a solid-state imaging device 10 offirst embodiment, FIG. 2 shows a cross-sectional view of the solid-stateimaging device 10 shown in FIG. 1 along line a-a, and FIG. 3 shows apartially-enlarged cross section view, illustrating the solid-stateimaging device 10 shown in FIG. 2.

The solid-state imaging device 10 includes an elongated substrate(molded case 18), a metallic layer 16 exposed in a surface of the moldedcase 18 and extending along an elongating direction of the molded case18, and an elongated solid-state image sensing element 20 mounted overthe metallic layer 16. The solid-state imaging device 10 has a long andnarrow geometry, having, for example, a length of about 55 mm, a widthof about 10 mm and a thickness of about 8 mm.

The molded case 18 has a form of a box which opens in the upper ends toform an upper opening, and such upper opening is closed with atransparent plate 22 such as a glass plate, thereby forming a hollowpackage form. A surface of the elongated metallic layer 16 is exposed ina substantially central portion of the upper surface in the interior ofthe molded case 18, and the solid-state image sensing element 20 ismounted over the surface of the metallic layer 16.

The metallic layer 16 is composed of a first metallic layer 12 and asecond metallic layer 14 as shown in FIG. 2, and it is designed that thefirst metallic layer 12 is exposed within the molded case 18. As shownin FIGS. 2 and 3, the second metallic layer 14 is adhered to a backsurface of the first metallic layer 12 in a region right under anamplifier unit of the solid-state image sensing element 20, and athickness of the metallic layer 16 in the region right under theamplifier unit is larger than thicknesses of other regions of themetallic layer 16.

The first metallic layer 12 contains copper as a major constituent, andfurther contains an alloy containing iron, phosphorus, tin or the like,and a layer thickness thereof is about 0.25 mm.

The second metallic layer 14 contains one or more metal(s) selected froma group consisting of copper, aluminum and an alloy thereof. In thepresent embodiment, oxygen-free copper may be employed as copper. Oxygenfree copper has better thermal conductivity, and thus is preferablyemployed. A thickness “a” of the second metallic layer 14 is about 1.5mm.

Typical solid-state image sensing element 20 employed here is aone-dimensional charge-coupled device (CCD), which includes an inputelectrode in one end and also includes an amplifier unit and an outputelectrode in another end, and also includes several-thousand pixelsarranged between both ends to form a straight line pattern. A heatgenerated from the solid-state image sensing element 20 concentrates onthe amplifier unit. A solid-state image sensing element that exhibits apower consumption of 0.4 W to 2 W may be employed as the solid-stateimage sensing element 20 employed in the present embodiment. Typicalsolid-state image sensing element 20 may be an elongated device having adimension of, for example, several tens mm in length, 0.3 to 1.2 mm inwidth and 0.3 to 0.7 mm in thickness.

A plurality of bonding pads (not shown) are included on the surface ofboth ends in elongating direction of the solid-state image sensingelement 20, and are electrically coupled thereto via a plurality ofinner leads (not shown) exposed within the molded case 18 and bondingwires.

Further, the inner leads extend through the molded case 18 and arecoupled to outer leads 30 outside of the molded case 18. A lead sectionis composed of the inner leads and the outer leads 30.

The solid-state imaging device 10 of the present embodiment may bemanufactured as follows. First of all, a strip of a metal such as copperalloy and the like is processed into a lead frame having a certaingeometry via a stamping process. The lead frame is composed of the firstmetallic layer 12 referred to as “island”, inner leads 34, outer leads30 and a frame. Subsequently, as shown in FIG. 4, a metallic plate isadhered so as to form a region right under the amplifier unit of thesolid-state image sensing element 20 to provide the second metalliclayer 14. The metallic plate may be obtained by profiling a strip of ametal such as oxygen free copper and the like via a stamping process.

An adhesive agent may be employed to adhere the metallic plate composedoxygen free copper or the like to the back surface of the first metalliclayer 12, or a metallic plate treated by a tin-plating and the backsurface of the first metallic layer 12 plated with gold may be adheredto the surface to be adhered via heating and compressing.

Then, the lead frame having the second metallic layer 14 formed thereinis stored in a certain metal mold, and then a molding process isconducted with an insulating thermosetting resin or the like to form themolded case 18. The molded case 18 has a form of a box which opens inthe upper ends, and the surface of the first metallic layer 12 isexposed in the inner surface thereof, and further, the surfaces of theinner leads 34 are exposed in vicinity of the end portion the firstmetallic layer 12.

Subsequently, the solid-state image sensing element 20 is adhered to asubstantially central portion in the surface of the first metallic layer12 exposed on the inner surface of the molded case 18 with athermosetting resin (die bond material) such as an epoxy resin, asilicone resin, a polyimide resin and the like.

Then, the inner leads 34 exposed in the molded case 18 are connected tothe bonding pads formed on the surface of the solid-state image sensingelement 20 via respective bonding wires.

After the inner leads 34 are connected to the bonding pads via thebonding wires as described above, a transparent plate 22 is pasted tothe open upper opening end of the molded case 18 to close the upperopening of the molded case 18. Further, unwanted sections of the leadframe are removed, and then a bending process for sections in the outerlead 30 is conducted to produce the solid-state imaging device 10 of thepresent embodiment.

The solid-state imaging device 10 of the present embodiment produced asdescribed above may be employed as a line sensor for electronicequipments having an imaging-ability such as a photo copying machine, ascanner and the like.

Advantageous effects obtainable by employing the above-describedconfiguration of the solid-state imaging device 10 of the presentembodiment will be described as follows. Since the metallic layer isformed to be thicker in the region right under the amplifier unit of thesolid-state image sensing element 20 according to the solid-stateimaging device 10 of the present embodiment, a heat generated in theamplifier unit can be released with an improved efficiency. Therefore,an increase in the temperature of the solid-state image sensing element20 is inhibited, and uniform temperature distribution of pixels isprovided, thereby achieving a stabilized imaging function.

Further, the present configuration allows providing benefits such as aninhibition of a warpage of the solid-state imaging device, a reductionin the manufacturing cost, a reduction in the product weight, asimplification of the manufacturing procedure and the like, as comparedwith the configuration, in which the metallic layer in the region rightunder the whole solid-state image sensing element extending over theoverall length thereof is formed to be thicker.

If the metallic layer is formed to be thicker in the region right underthe whole solid-state image sensing element extending over the overalllength thereof, a considerable difference in thermal expansion is causedfor the metallic layer and the mold resin, leading to possibly cause awarpage in the solid-state imaging device during the manufacture and inoperation. On the contrary, the configuration of having locally thickermetallic layer achieves minimized influence of the difference in thermalexpansion, so that a warpage of the solid-state imaging device can beprevented, thereby providing an improved production yield and anenhanced resolution of the imaging product.

Further, the metallic layer 16 may be composed of the first metalliclayer 12 and the second metallic layer 14, a multiple-layered structureof the first metallic layer 12 and the second metallic layer 14 may beformed in the region right under the amplifier unit of the solid-stateimage sensing element 20, and a single layer structure composed of thefirst metallic layer 12 may also be employed in other regions.

Even if a type of metal exhibiting lower strength lower elastic modulusand having higher thermal conduction, which is not suitable for amaterial of the lead frame, is employed, the above-describedconfiguration allows such type of metal being employed for the secondmetallic layer 14, such that the solid-state imaging device exhibitingfurther improved heat release-ability can be presented.

Further, since procedure of adhering the second metallic layer 14 to theback surface of the first metallic layer 12 provides themultiple-layered structure of the metallic layer 16 in the presentembodiment, the simple procedure for producing of the device can beachieved, as compared with the case of producing the multiple-layeredstructure by employing a single-piece molding process of metalliclayers. Further, since the metallic composition of the second metalliclayer 14 can be selected to be different from that of the first metalliclayer 12, an enhanced degree of flexibility in the design of the devicecan be achieved.

In addition, the thermal resistance of the solid-state imaging device 10can be selected to be equal to or lower than 35 degree C./W, and thethickness of the second metallic layer 14 can be selected to be equal toor larger than 1 mm.

This allows obtaining the above-described advantageous effects, andfurther, even if the second metallic layer 14 is provided, an increasein the temperature of the solid-state imaging device 10 due to a heatgenerated in the solid-state image sensing element can be fallen withinan allowable range, thereby further effectively inhibiting adeterioration in the imaging function of the solid-state imaging device.

For example, since the temperature in the electronic equipment employingthe solid-state imaging device 10 is generally elevated up to atemperature of around 65 degree C., the solid-state imaging device 10 isrequired to have a thermal resistance of equal to or lower than 35degree C./W for the solid-state imaging device 10 of the powerconsumption of 1 W, so that the upper limit temperature in use of thesolid-state imaging device 10 is equal to or lower than 100 degree C.Experiments were conducted for obtaining a relationship of the thermalresistance of the solid-state imaging device 10 with the thickness ofthe second metallic layer 14 in the following condition. Results of suchexperiments are shown in table of FIG. 5.

Length of solid-state imaging device 10: 70 mm, Width: 9 mm;

Material of the first metallic layer 12: copper alloy (K F C:commercially available from Kobe Steel, Ltd.),

Thickness: 0.25 mm;

Material of the second metallic layer 14: oxygen free copper, Thermalconductivity: 391 W/mk;

Length of the solid-state image sensing element 20: 49 mm, Width: 0.8mm, Power consumption: 1 W; and

Mount materials (for providing adhesion of the first metallic layer 12to the solid-state image sensing element 20): three types of materialshaving thermal conductivities of 4 W/mk, 2 W/mk and 1.2 W/mk wereemployed.

The results of the experiments indicate that the thermal resistance ofthe solid-state imaging device 10 can be designed to be equal to orlower than 35 degree C./W regardless of the thermal conductivity of themount material, by selecting the thickness of the second metallic layer14 to be equal to or larger than 1 mm. It is confirmed that, having suchconfiguration, an increase in the temperature due to a generation ofheat from the solid-state image sensing element 20 can be inhibited,thereby more effectively inhibiting a deterioration in the imagingfunction. In addition to above, the upper limit of the thickness of thesecond metallic layer 14 may be sufficient to be within an allowablerange in the design of the device, and for example, may be not largerthan 2 mm. In addition to above, even if the solid-state imaging device10 is designed to have higher upper limit temperature in the operationfor accommodating the increased temperature in the electronic equipmentemploying the solid-state imaging device 10, the results as describedabove can also be obtained.

Second Embodiment

Descriptions of a solid-state imaging device 10 of the second embodimentwill be made only on features that are different from the solid-stateimaging device 10 of first embodiment in reference to annexed figures,and the similar features will not be repeated.

The solid-state imaging device 10 of second embodiment includes twoprotruding portions 24 on a surface of a first metallic layer 12 inother regions except a region right under the amplifier unit of thesolid-state image sensing element 20. The solid-state image sensingelement 20 is partially mounted over the first metallic layer 14 in aregion right under the amplifier unit, and is also partially mountedover protruding portion 24 in the other regions. This allows maintainingthe solid-state image sensing element 20 to be substantially horizontal,thereby providing a stabilized imaging function of the solid-stateimaging device 10.

The protruding portions 24 may be formed of a molding resin. Heights ofprotruding portions 24 from the surface of the first metallic layer 12is determined in consideration of a cure shrinkage of the employingmolding resin.

The solid-state imaging device 10 of the present embodiment may be alsomanufactured in the similar manner as in first embodiment, except that amolding process for the protruding portions 24. In order to form theprotruding portions 24, a metal mold having cavities corresponding tothe protruding portions 24 may be employed.

Advantageous effects obtainable by employing the above-describedconfiguration of the solid-state imaging device 10 of second embodimentwill be described as follows. When the second metallic layer 14 isprovided in the region right under the amplifier unit of the solid-stateimage sensing element 20 as in first embodiment, levels of thermalshrinkage and cure shrinkage of the resin caused during the moldingprocess are different for locations provided with the second metalliclayer 14 and for locations without additional metallic layer. Morespecifically, the locations provided with the second metallic layer 14(resin is thin) exhibits smaller shrinkages due to a smaller amount ofthe resin, and the locations without the second metallic layer 14 (resinis thick) exhibits larger shrinkages due to a larger amount of theresin. Thus, the height of the surface of the first metallic layer 12may possibly be partially variable. However, such variation in theheight is quite small, and thus does not cause a problem in general.

Nevertheless, when further improved accuracy is required for the imagingfunction of the solid-state imaging device 10, it is preferable toreduce the variation in the height of the surface of the first thismetallic layer 12.

The solid-state imaging device 10 of the present embodiment isadditionally provided with the protruding portions 24 on the surface ofthe first metallic layer 12, which function as compensating thedifference in the amounts of the thermal shrinkage and the cureshrinkage of the resin. Therefore, even if the solid-state image sensingelement 20 is mounted on the surface of the first metallic layer 12, thesolid-state image sensing element 20 is maintained to be substantiallyhorizontal, and thus a warpage and/or waviness due to the difference inthe height of the surface of the first metallic layer 12 are notgenerated. Therefore, more stable imaging function of the solid-stateimaging device 10 is achieved, thereby allowing an imaging with animproved accuracy.

While the above-described embodiments illustrate the exemplaryimplementations of the present invention in reference to the annexedfigures, various modifications other than that disclosed above may alsobe available.

While the present embodiment has been described in reference to theexemplary implementation that is provided with the amplifier unit of thesolid-state image sensing element 20 in only one side end, the thicknessof the metallic layer 16 in the regions right under the amplifier unitof both ends may be larger than the thickness of the metallic layer 16in other regions in a case of having the amplifier units in both ends.

While the present embodiment has been described in reference to themetallic layer 16 having the multiple-layered structure composed of thefirst metallic layer 12 and the second metallic layer 14, the metalliclayer 16 may alternatively be a single member that is formed by asingle-piece molding process of these metallic layers.

The second metallic layer 14 may be composed of a plurality of layershaving different metallic compositions. While the solid-state imagingdevice 10 of second embodiment includes two protruding portions on thesurface of the first metallic layer 12, at least one protruding portionor more is sufficient to be included provided that the solid-state imagesensing element 20 can be substantially horizontally maintained.

In addition, the present embodiment can be configured that the bottomsurface of the metallic layer 16 is present in the same plane as thebottom surface of the molded case 18. This allows the bottom surface ofthe metallic layer 16 to be in contact with the base substrate, when thesolid-state imaging device 10 is mounted, thereby providing an improvedheat release-ability.

The height of the metallic layer 16 from the upper surface of the moldedcase 18 may be designed to be higher than conventional solid-stateimaging devices. For example, a height of around 2 mm may be employed.This allows preventing a defective characteristic such as pixel faultscan be inhibited, even if the trash is adhered to the surface of thetransparent plate 22.

In addition, in the substantially central portion in the elongatingdirection of the molded case 18, a resin layer having the upper surfacethat is higher than the surface (sensor surface) of the solid-stateimage sensing element 20 may be provided between the interior wall ofthe molded case 18 and the solid-state image sensing element 20. Thisallows reducing failure in the imaging due to a disturbance of ambientlight.

It is apparent that the present invention is not limited to the aboveembodiment, and may be modified and changed without departing from thescope and spirit of the invention.

1. A method of manufacturing a solid-state imaging device whichcomprises a first metallic layer, a solid-state image sensing elementwhich includes an amplifier unit therein mounted on an upper surface thefirst metallic layer, and a second metallic layer adhered to a lowersurface of the first metallic layer, the method comprising: plating atleast a region of the lower surface of the first metallic layer with onemetal selected from a metal group consisting of gold and tin so that theregion is located right under the amplifier unit of the solid-stateimage sensing element; plating a top surface of the second metalliclayer with another metal selected from the metal group consisting ofgold and tin; and adhering the top surface of the second metallic layerplated with the other metal to the region of the lower surface of thefirst metallic layer plated with the one metal via heating andcompressing to form a gold-tin alloy made from the metal groupconsisting of gold and tin.
 2. The method as set forth in claim 1,wherein the first metallic layer is plated with gold and the secondmetallic layer is plated with tin.
 3. The method as set forth in claim1, wherein the first metallic layer is plated with tin and the secondmetallic layer is plated with gold.
 4. The method as set forth in claim1, wherein the first metallic layer contains at least one metal selectedfrom a group consisting of copper, aluminum and an alloy thereof.
 5. Themethod as set forth in claim 1, wherein the second metallic layercontains at least one metal selected from a group consisting of copper,aluminum and an alloy thereof.
 6. The method as set forth in claim 1,wherein a thickness of the second metallic layer is equal to or largerthan 1 mm.
 7. The method as set forth in claim 1, wherein the firstmetallic layer includes a depressed portion at a region except theregion right under the amplifier unit, at least one protruding portionis formed on a top surface of the depressed portion of the firstmetallic layer, the solid-state image sensing element is partiallymounted on the first metallic layer in the region right under theamplifier unit and is also partially mounted on the protruding portion,so that the solid-state image sensing element is substantiallyhorizontally maintained.